In HPLC, a sample that is stored at atmospheric pressure is generally conveyed with the aid of a metering pump, in an automated method (autosampler), into a sample loop which is connected at both ends to what is called a shear valve. When the desired amount of the sample is located in the loop, the shear valve is switched such that the solvent stream now flows through the sample loop, and the sample can thus be conveyed at high pressure through the separating column. This procedure by which the sample to be analyzed is fed or introduced into the solvent stream is called (sample) injection.
However, particularly at high operating pressures of a chromatography system, shear valves can be switched only with high drive moments. The rubbing and sealing shear surfaces are therefore subject to a high degree of wear and have to be frequently replaced. Despite the high sealing normal force, the positioning of the two shear surfaces relative to each other also requires a high level of precision. The shear surfaces are also very sensitive to abrasive constituents in the solvent stream or sample stream. Upon switching of the shear valves, the flow or the volume flow through the column is interrupted for the switching time, such that the pressure upstream of the separating column drops and the pressure in the high-pressure pump rises. In particular, the pressure drops particularly strongly when the sample loop, at atmospheric pressure after suction of the sample, is introduced into the feed line to the column, and the high-pressure pump has to compress the content of the sample loop to the operating pressure upstream of the column. During this time, the direction of flow in the feed line to the separating column reverses, which reduces the useful life of the separating column and has a negative effect on the separation efficiency.
DE 10 2008 006 266 B4 has already disclosed an arrangement which, with the aid of a shear valve, is able to completely eliminate the fluctuations in pressure that occur during injection according to the prior art.
However, manual injections of samples, of the kind described in the patent documents U.S. Pat. No. 3,985,166 and U.S. Pat. No. 3,940,994 for example, are unsuitable for an automated method in what is called an autosampler, with different samples having to be collected from several vials or several sample reservoirs. Moreover, during such an injection, the sample is diluted in the solvent stream, which results in poor sample detection.